There are a variety of methods of molding articles. These include autoclaves, hydroclaves and compression molding. Particularly for high temperature molding (e.g., polyimide resins) one isostatic molding process has shown great advantage over other methods. This method is described in commonly assigned European Patent Application No. 87630010.4 entitled "Molding Method and Apparatus Using a Solid, Flowable, Polymer Medium" to Kromrey. An article is molded by contacting it with a solid polymer medium, such as an especially low strength unfilled silicone rubber which is solid and able to flow readily. Thermal expansion of the medium or mechanical force is used to create molding pressure and thereby provides a substantially uniform pressure on the article precursor. Various temperature and pressure cycles can be attained; constant high pressures can be maintained on the article precursor during cooldown, optionally aided by flowing of medium to and from a vessel in which the article precursor is being molded. The method is particularly adapted to molding filler or fiber reinforced thermosetting polymer composite articles.
In addition, the process may be used at higher temperatures for the molding of glass matrix (compglas) composites, etc. Molding at very high temperature, under isostatic pressure conditions, presents difficult load transfer problems. Molding an article against a forming mandrel or tool requires an even application of pressure onto the surface to ensure proper thickness control. If the pressure is not substantially even, the matrix material of the composite can be forced into a lower pressure region. This results in poor distribution of fibers and matrix material.
Application of isostatic pressure by the thermal expansion of a polymeric medium has been effective at temperatures under 1000.degree. F. If higher temperatures are used, the media must be protected by an insulative layer of thermally resistant material. The thickness of this layer is dictated by the temperature drop required to protect the organic medium. If this layer becomes too thick, it cannot properly transmit the molding pressure to surfaces which are not substantially flat.
Thus, there has been a continuing search in this field of art methods of transferring pressure at high temperatures.